The arrival of five healthy lambs has demonstrated that eggs at an earlier stage of development than previously used can be coaxed to maturity in the laboratory and go on to produce viable offspring. The work marks the first time this specific technique has succeeded in a large mammal, opening a path that could enlarge the pool of usable eggs available for fertility treatment.
Scientists at the University of Leeds, led by Helen Picton, collected immature ovarian follicles from sheep. They matured the follicles in the laboratory with a combination of reproductive hormones, including follicle-stimulating hormone, together with growth factors. Roughly 60 percent of the treated follicles developed into mature eggs. Around 30 percent of those eggs were successfully fertilised, creating embryos that were transferred to the uteruses of 18 recipient sheep. Five healthy lambs resulted.
One of the female lambs later produced two offspring of her own, confirming the long-term fertility of animals conceived through the method. The first lamb was born in early 2024, with four more births following in 2026. New Scientist revealed that five healthy lambs have been born from an experimental IVF procedure using in-vitro maturation of highly immature eggs collected from sheep, with the first birth in early 2024, four more in 2026, and one of the resulting ewes later producing two lambs of her own.
A comparable technique produced live births in mice more than 30 years ago. The leap to sheep is significant because their reproductive physiology sits closer to that of humans than the mouse model does. The findings were presented at the European Society of Human Reproduction and Embryology annual meeting in London on 7 July 2026.
Practical gains for fertility treatment
The advance could increase the number of usable eggs for fertility treatments and improve success rates, particularly for patients with damaged ovaries. Young cancer patients who have ovarian tissue cryopreserved before treatment stand to gain most, as the new protocol targets follicles at an earlier stage than the somewhat immature eggs already used in limited cases in human clinics.
Further studies on human eggs are expected in the coming years. Small trials leading to live births could become possible within five to 10 years, subject to ethical approval. Throughout, the emphasis remains on rigorous safety data gathered step by step.